The present invention relates to an impedance network including a source having a DC component and an AC component, a variable resistance, a DC detector circuit to ascertain changes in the value of said resistance, terminals of all three means being coupled to distinct ports of a multi-port network, and a filter circuit associated to said DC detector circuit to substantially prevent said AC component from having an influence on the operation of said DC detector circuit which has at least one further terminal at a fixed DC potential.
Such a network is already known from Belgian Pat. No. 688 760 (E. HERTER). This known network is a telephone ring-trip circuit arranged in a telephone exchange and including a multi-port network comprising a bridge circuit with a first port able to be connected to a telephone subscriber station via a telephone line, with a second port connected to a source having a DC component and an AC or ringing component and with a third port, the terminals of which are connected to the base and emitter of a normally non-conductive transistor DC detector having a collector or further terminal at a fixed potential. The base and emitter of this transistor DC detector are coupled to ground via respective (shunt) capacitors which form together with (series) resistances of the bridge circuit a filter circuit for the AC or ringing component.
After a call initiated by a telephone subscriber has been detected in the telephone exchange and the dialled telephone number of a called subscriber has been obtained, the above mentioned source is connected via the bridge circuit to a telephone line leading to the called subscriber station and a ringing device is operated therein by means of the AC or ringing component applied thereat. When the called subscriber subsequently takes off his telephone handset a line loop including the called subscriber station and the telephone line is closed from a DC viewpoint so that the resistance connected to the first port of the transistor DC detector is sharply reduced. As a consequence the transistor DC detector then becomes conductive, thus indicating that ringing can be terminated. In principle, the filter circuit operates in such a way that the ringing component has no influence on the condition of the detector transistor. However, it has been experienced that due to tolerances on the values of the capacitors and the resistances the filtered AC signals applied to the base and emitter electrodes of the transistor DC detector may have such a phase shift and amplitude that the transistor can be instantaneously switched on and off under the sole influence of these AC signals. This is obviously a drawback because the operation of the detector circuit should be only dependent on the DC variations at its input port.